Lower limb plyometric exercises combine speed and strength to produce an explosive-reactive movement (14,48). These exercises involve a cycling of eccentric (stretch) and concentric (shortening) muscle contractions generally using the body as an overload stress (13,42). Plyometric exercises use accumulated elastic energy during the eccentric phase of the contraction to help augment the concentric phase (25). Furthermore, the rapid stretch-shortening cycle (SSC) plyometric activity is purported to cause an excitation of the muscle reflexes (26). The flight phase with repeated jumps is characterized by an electromyographic (EMG) preactivity in the muscle (40). These training exercises are used to improve the performance of other sport movements such as jumps, sprinting, and agility.
Plyometric activities can include drop jumps (DJ), hops, jumps over hurdles, bounding, countermovement jumps (CMJ), and others. Generally, power training programs use a variety of these plyometric exercises, which may exceed 20 types a year (34). Coaches typically use jump drill training with a short contact time and maximal height or maximal velocity to induce changes in neuromuscular power performance (6,10,19,30,39,54,55). Although there are a great number of plyometric exercises, little is known about quantifying drill intensity and the optimal technique for an effective plyometric training program (17). Accordingly, an optimal explosive-ballistic training exercise is a movement that uses similar contact time and force production as specific sport actions (4). With plyometric techniques, coaches attempt to increase explosive power, jump height and shorten contact time, in the hope that this training will be sport specific. For training specificity (i.e., velocity, contraction type, and training mode specificity) (4), it is important to know which exercises have the greatest power output or what landing technique must be used to induce changes in specific sport actions.
The current plyometric classifications do not recommend an appropriate or optimal landing technique for plyometric drills. However, research has shown that landing on different parts of the foot can generate various amounts of power (6,27,33). Bobbert et al. (6) showed that when a DJ was performed with a rebound technique (using just the forefoot), the force generated (4,099 N) was higher than in a DJ with a slow countermovement technique (2,649 N) or in a traditional CMJ (2,094 N) (6).
It has been shown that a change of technique when performing DJs and hopping alters many kinematic and kinetic variables (1,7,18,27). Jumps over obstacles are one of the most common types of plyometric drills used to increase power (2,39,41,51). However, it is not known if a change of landing technique can generate changes in other variables in a similar form to those observed during DJs. It may be possible with a simple technique instruction to positively modify training quality or specificity.
Thus, the main objective of this study was to compare the neuromuscular characteristics of 2 types of jumps: hurdle and DJ. In addition, 3 types of landing techniques were investigated: preferred (instructions to jump with a technique allowing the athlete to jump as quickly and as high as possible), flat foot, and forefoot technique. It was hypothesized that hurdle jumps would be more powerful than DJs and that the flat foot technique would diminish mechanical power.
Experimental Approach to the Problem
Three landing techniques (preferred, flat foot, and forefoot technique) were performed during DJs and hurdle jumps, and reaction forces, contact time, rate of force development (RFD), and lower limb EMG activity were measured. The first visit to the laboratory included general anthropometric measurements and an orientation to the jumps and tests procedures. The second visit to the laboratory was to assess jumps. During the experimental testing period, a general warm-up was followed by 2 trials of a maximum CMJ on a force platform. After calculating the maximum height reached during CMJs, the athletes performed in a random order, a series of DJs and jumps over hurdles bilaterally. The athletes performed DJs from a box and landed on the force platform. The athletes also jumped forward over 2 hurdles without stopping with the force plate positioned between the 2 hurdles. The height of the DJ and the hurdle height were set at 100% of the maximum height reached with the CMJ.
Twenty-five male athletes (age 21.1 ± 4.5 years, weight 83.4 ± 10.9 kg, height 184.4 ± 8.1 cm, and training experience 6.3 ± 4.1 years) from the Memorial University (9 volleyball players, 5 soccer players, 5 basketball players, and 6 well-trained recreational athletes) were assessed. The data were collected during the athletes' competitive season. They refrained from performing any type of exercise 24 hours from the testing sessions. The subjects were informed of all the procedures and provided written consent to participate in this study in accordance with the Memorial University of Newfoundland Human Investigation Committee. All the athletes (including the recreational) regularly resistance trained and were accustomed to performing multiple plyometric drills. The typical volume of plyometric training performed by the athletes was >100 repetitions per session, and drills were generally performed as fast as possible unilaterally and bilaterally minimizing contact time. However, none of the subjects had experience performing DJs. Because the subjects were not familiar with DJs, this test was the first assessed jump to avoid fatigue. Exclusion criteria comprised any musculoskeletal injuries and any pain, which did not allow the athlete to jump properly.
All the experimental sessions were performed between 10:00 and 14:00 hours. The athletes were hydrated ad libitum before and during the test and wore their regular training shoes. All the jumps were assessed with the arms akimbo. The warm-up consisted of 10 minutes of cycling at an intensity of 75 W–60 rpm followed by 5 sets of 5 submaximal hopping, 5 single submaximal CMJ and 2 maximal CMJ. Only dynamic stretching exercises were allowed during the warm-up to avoid muscular power deficits associated with static stretching (3,46).
Then, the subjects stood on the force platform and were asked to perform a maximal CMJ. The subjects did not receive any specific instructions about leg position or knee movement during the jump. Two trials were tested with 1-minute rest to avoid fatigue. The maximum CMJ height was used to establish DJ and hurdle jump height. The average flight time of the 2 trials of each jump was used to calculate jump height.
Because the CMJ does not involve a rapid SSC movement, which differs from the hurdle jump (8), the analyses did not consider differences between CMJ and the other 2 jumps. The order of DJ and hurdle jump tests was randomized with 5 minutes of rest between DJ and hurdle jumps.
For the DJ, the athletes stood on the edge of a box. The athletes were instructed to start the drop by leaning forward at takeoff and land bilaterally with both feet at the same time. For the preferred technique, the athletes received the following instructions: “jump as quickly and as high as possible.” The subjects used the technique they believed was their best to reach the objective. For the FLAT technique athletes were asked to perform the same DJ but land on a flat foot (DJFLAT), with the same objective as before. Finally, the athlete was asked to land just with the forefoot (DJFORE). The subjects were not allowed to touch the floor with the heel during this technique. Two evaluators strictly observed the landing technique. In general, the preferred technique was similar to the FORE. The difference resided in the fact that with the preferred technique, the athlete was not instructed to focus or consciously perform a specific technique. The preferred technique focused on the outcome (jump as quickly and as high as possible) rather than on the technique. However, because of the jumping instructions, the preferred technique never closely resembled the FLAT technique.
The athletes also jumped forward over 2 hurdles with the force plate positioned between hurdles. Hurdles were spaced apart as the athletes requested. For all types of landings, the athletes were instructed to jump with 2 legs as fast as possible with no hesitation (feet shoulder width apart). For the preferred technique, the athletes repeated the same procedure as during the DJ. The athletes performed a flat foot landing touching the heel to the floor (HJFLAT) and a forefoot landing technique with no heel supporting (HJFORE). The instruction for the hurdle jumps was to jump as fast as possible but not as high as possible. The takeoff was strictly monitored with no intermediate jumps or delays during the eccentric-concentric transition phases. Two trials for each jump type were assessed.
An AMTI force platform (400 × 600 × 83 mm, model BP400600 HF-2000, Watertown, MA, USA) was used to evaluate the different jumps using 2 axes. The force platform was connected to an amplifier (AMTI Miniamp MSA-6–Gain 2000). NIAD software was used to collect data with a sample frequency of 2,000 Hz. The force platform was calibrated by using the shunt technique provided by the company.
The EMG surface electrodes (MediTrace 133, Kendall, 1-cm silver/silver chloride—Budlow Technical products, Toronto, Ontario, Canada) were placed on the midbelly of the rectus femoris, biceps femoris, tibialis anterior, and gastrocnemius. Three of the 4 muscles were biarticular, and all muscles would be involved in a coordinated fashion during the takeoff and landing components of the jumping techniques (i.e., hip flexion and extension: rectus femoris and biceps femoris, knee flexion and extension: biceps femoris and rectus femoris, plantar flexion and dorsiflexion: gastrocnemius and tibialis anterior). The EMG activity was monitored and collected at 2,000 Hz, amplified (bipolar differential amplifier, input impedance = 2 MΩ, common-mode rejection ratio >110 dB min [50/60 Hz], gain ×1,000, noise >5 μV), and analog to digitally converted with a 12-bit acquisition and analysis system (Biopac Systems, Santa Barbara, CA, USA). The skin surface was prepared by shaving the zone, cleaning with alcohol, and removing dead epithelial cells with abrasive sandpaper. The EMG signal was analyzed in 3 segments. It was analyzed 100 milliseconds before the start of the jump (PRE), during the entire duration of the eccentric and concentric phases.
Contact time (milliseconds) for the jumps was defined as the sum of the eccentric and concentric phases. The velocity was calculated by the following formula: Velocity (meters per second) = F z × Δt/m and was used to define the eccentric and concentric phases. The F z represents the vertical ground reaction force (VGRF), Δt is the time period, and m is the athletes' mass. The eccentric phase was considered as a negative velocity and concentric phase as a positive velocity. Peak VGRF was the maximal force produced during the jump. Flight time was defined as the time the athlete stayed in the air after the jump.
Different analytical methods were used to identify the eccentric and concentric phases for the DJs and hurdle jumps. The same formula used for CMJ analysis cannot be implemented for DJ, because DJs do not start with a zero velocity. Therefore, the Voigt method was used to calculate the velocity and determine eccentric and concentric phases (50). Takeoff velocity was estimated from flight time with the following equations:
During the following integration of force, the order of the movements that actually happened was reversed corresponding to a backward integration with respect to time. Finally, the following equation was used:
The touchdown represented the time when the toes touched the ground and takeoff the time when the toes left the ground. The F z represents the VGRF, g is the acceleration due to gravity, and BM is the athletes' mass.
Sometimes DJs or hurdle jumps display 2 separate force peaks. A 2-peak force curve is typical of a FLAT landing technique. This curve is similar to other motor actions reported in the literature (5,9,12). Because the hurdle jump method in this study did not have the subject land on the force platform after the initial contact, the Voigt method was not applied. Alternatively, the Cormack method (12), which considers the end of the eccentric phase as the minimum VGRF after the first peak force, was used. This point can be found after the first force peak, which is considered the weight absorption and is represented by a high frequency and passive force (35).
Single peak force curves were the typical shapes for the FORE and preferred landing techniques. During the single peak curve, the maximal value represents the separation of eccentric and concentric phases. This condition was controlled later with a goniometry (PS2137 Roseville, CA, USA) during these types of jumps (data not published).
Raw EMG signals were amplified (1,000) and bandpass filtered (Blackman 62 dB, 10–500 Hz). The signal was integrated, and the root mean square was calculated (AcqKnowledge Software; Biopac Systems). To analyze the EMG activity during DJ and hurdle jump, 3 periods were taken into account. One hundred milliseconds before the contact time was considered the prelanding phase. The full duration of the eccentric and concentric phase time were also analyzed.
Rate of Force Development
The average RFD was considered as the peak force developed during the concentric portion of the contraction divided by the time employed (newtons per second) (31). The average RFD was calculated as follows: newtons per second = (AFCON − body weight [newtons]/CCT), where AFCON is the average force applied during the concentric phase, and CCT is the time during the concentric phase of all the jumps.
Vertical Leg Stiffness
Another important variable used to understand the differences in jump performance was vertical stiffness (32). The following formula was used:Vertical Stiffness (kilonewtons per meter) = body mass × w 2, where w is the natural frequency of oscillation.
The statistical analyses were completed using SPSS 17.0 for Windows (SPSS, Inc., Chicago, IL, USA). A 3-way analysis of variance 2 × 3 × 3 with repeated measure was employed to test for main effects between type of jump (DJ and hurdle jump), landing (preferred, FLAT, and fore), and time (prelanding, eccentric, concentric) during the jump. Significant main effects were further analyzed with Bonferroni-adjusted pairwise comparison of within-subject differences among the variables. The criterion for significance was set at a level of F-ratios of p ≤ 0.05. Effect sizes (ES = mean change/SD of the sample scores) were also calculated and reported (11). Cohen applied qualitative descriptors for the ESs with ratios of <0.41, 0.41–0.7, and >0.7 indicating small, moderate, and large changes, respectively. All data are reported as mean ± SD.
The condition of sphericity was met in all conditions.
Total Contact Time
There was a significant main effect for contact time and type of landing. The hurdle jump had a 36.9% shorter contact time compared with that of the DJ (p < 0.001, F = 75.8, ES = 2.94; Table 1). The preferred technique had a 29.1% shorter contact time than did FLAT (p < 0.001, F = 28.5, ES = 1.51) and 9.6% longer contact time than did the FORE technique (p < 0.05, F = 28.5, ES = 2.06). Finally, FLAT type of landing had a 25.9% longer contact period than did FORE (p < 0.001, F = 28.5, ES = 2.5). Jump and landing type interactions showed a significant 23.8% shorter ground contact time for DJ FORE vs. FLAT (p < 0.001, F = 48.5, ES = 2.3).
Vertical Ground Reaction Forces
Main effect differences showed that hurdle jump forces were 11.0% higher than DJ (p < 0.001, F = 18.0, ES = 1.71; Table 1). Main effects for type of landings revealed that FLAT technique showed 30.8% less reaction force than the preferred technique did (p < 0.001, F = 64.9, ES = 1.33) and 40.9% less than FORE techniques did (p < 0.001, F = 64.9, ES = 1.44). Meanwhile, significant interactions showed that, the DJ preferred technique had a significantly higher force (14.9%) than DJ FLAT did (p < 0.05, F = 69.5, ES = 2.14). Moreover, DJ FLAT showed the lowest force level and had substantial differences with all the other jump categories. The DJ FLAT was 25.9% lower than DJ FORE (p < 0.001, F = 69.5, ES = 1.35).
Rate of Force Development
The main effects for the hurdle jump demonstrated a 46.3% higher RFD than DJ (p < 0.001, F = 44.4, ES = 1.21; Table 1). When type of landing was considered, the FORE technique was 11.3% higher than the preferred technique (p < 0.001, F = 60.1, ES = 2.03) and 45.0% higher than FLAT was (p < 0.001, F = 60.1, ES = 3.24). Furthermore, the preferred technique was 38.0% higher than FLAT as well (p < 0.001, F = 60.1, ES = 2.85). Analyzing all jump categories, the DJ preferred technique had an RFD 35% greater than that of DJ FLAT (p < 0.05, F = 41.4, EF = 2.21). Moreover, DJ preferred showed a 40.9% lower RFD than did the HJ preferred technique (p < 0.001, F = 41.4, ES = 1.00) and 43.6% lower than HJ FORE (p < 0.001, F = 41.4, ES = 0.96). The DJ FLAT had the lowest value for RFD representing 41.6% of DJ FORE (p < 0.001, F = 41.4, ES = 0.98), 56.4% of hurdle jump preferred (p < 0.001, F = 41.4, ES = 0.75), and 58.4% of hurdle jump FORE (p < 0.001, F = 41.4, ES = 0.72). Finally, the RFD during DJ FORE was 25.3% lower than in the hurdle jump preferred technique (p < 0.001, F = 41.4, ES = 1.31) and 28.7% lower than in hurdle jump FORE (p < 0.001, F = 41.4, ES = 1.25).
Concerning leg stiffness, the main effects for types of jumps and landing techniques were discovered (Table 1). Hurdle jumps were 64% stiffer than DJ (p < 0.001, F = 131, ES = 4.8). The preferred technique was 42.4% greater than FLAT (p < 0.001, F = 96, ES = 3.00). Significant interactions for all the jump categories showed that the DJ preferred technique was 32.8% stiffer than DJ FLAT (p < 0.05, F = 138, ES = 2.39) but was 204, 66.4, and 221% less stiff than the hurdle jump preferred technique (p < 0.001, F = 138, ES = 0.55), hurdle jump FLAT (p < 0.05, F = 138, ES = 0.96), and hurdle jump FORE (p < 0.001, F = 138, ES = 0.52), respectively. The DJ FLAT exhibited the least stiff technique. The DJ FLAT was 76.0, 354, 147.9, and 379% less stiff than DJ FORE (p < 0.001, F = 138, ES = 0.94), hurdle jump preferred (p < 0.001, F = 138, ES = 7.75), hurdle jump FLAT (p < 0.001, F = 138, ES = 4.03), and with hurdle jump FORE (p < 0.001, F = 138, ES = 8.2), respectively.
To illustrate more clearly which variables had the best performance during different jumps, the differences can be appreciated by viewing Table 2, which shows the optimal result as 100% (bold font) in every variable (column) with the remaining variables relative (percentage of) to the optimal condition.
Rectus Femoris Electromyographic Activity
The main effects for rectus femoris EMG activity were found for type of jump, landing, and contact phase (Tables 3 and 4). Hurdle jump showed a 30.0% higher EMG activity than did DJ (p < 0.001, F = 28.2, ES = 2.49). The preferred technique had an 18.4% lower activity than FLAT did (p < 0.01, F = 11.9, ES = 1.46). Meanwhile, the FLAT technique had a 47.0% higher activity than the FORE technique (p < 0.01, F = 11.9, ES = 2.14).
The main effects for the contact phase showed that prelanding had a 275 and 204% less activity than did eccentric (p < 0.01, F = 95.4, ES = 0.46) and concentric phases (p < 0.001, F = 95.4, ES = 0.57), respectively. Finally, the eccentric phase showed 18.8% more EMG activity than did the concentric phase of contraction (p < 0.01, F = 95.4, ES = 2.12).
Biceps Femoris EMG Activity
The main effects for the biceps femoris EMG activity were found for type of jump, landing, and contact phase (Tables 3 and 4). The DJ had a 68.8% less EMG activity than hurdles did (p < 0.001, F = 24.0, ES = 0.89). The preferred technique had an 18.9% more EMG activity than FLAT did (p < 0.01, F = 23.0, ES = 1.90). Finally, prelanding contact phase showed a 135 and 212% less activity than did eccentric (p < 0.01, F = 23.0, ES = 0.65) and concentric phases (p < 0.001, F = 23.0, ES = 0.49), respectively. The eccentric phase showed a 32.7% lower EMG activity than did the concentric phase of contraction (p < 0.001, F = 23.0, ES = 1.16).
Tibialis Anterior EMG Activity
The main effects for tibialis anterior were found for type of landing and contact phase (Tables 3 and 4). The preferred landing technique had a 36.4% less EMG activity than did FLAT (p < 0.001, F = 23.0, ES = 1.24) but a 21.1% higher activity than FORE technique did (p < 0.001, F = 23.0, ES = 2.18). The prelanding contact phase showed 54.5% less activity than eccentric phase did (p < 0.01, F = 23.0, ES = 1.08). The eccentric phase exhibited a 22.2% higher activity than CON did (p < 0.05, F = 23.0, ES = 2.12).
Gastrocnemius EMG Activity
The main effects for the gastrocnemius showed statistical differences for the type of landing and the contact phase (Tables 3 and 4). The preferred technique had a 26.3% more activity than FLAT did (p < 0.001, F = 29.8, ES = 2.27). In addition, the FLAT technique demonstrated a 47.0% lower activity than FORE did (p < 0.001, F = 29.8, ES = 1.13). When the phases were analyzed, prelanding showed 54.6 and 73.3% less activity than did the eccentric (p < 0.01, F = 17.9, ES = 1.09) and concentric phases (p < 0.001, F = 17.9, ES = 0.99), respectively.
The most important finding in this study was that the FORE and preferred hurdle jump techniques were more powerful plyometric activities than was DJ (shorter contact time, higher VGRF, RFD, and leg stiffness). The second major finding was that FORE and preferred landing provided the best results for all mechanical power variables. Preferred and FORE hurdle jumps developed the greatest mechanical power in this study. The literature provides conflicting findings with a number of publications in agreement with the findings of this study (2,6,17,27,53), whereas others recommended DJ as the highest intensity type of jump (38,47). The conflict in the literature may be related to the drop heights. For example, drop heights of 20–60 cm were employed even when the subjects had different training backgrounds (6,27,53). It is clear that the drop height condition affects the results (36,51). This study used an average drop height of 34.4 cm corresponding to 100% of the maximal CMJ height. The literature shows several studies with DJ heights greater than the CMJ maximal height (25,28,40,45,50). Such heights can generate reflex inhibition (20,43).
To plan and implement DJ training, it is important to determine the optimal drop height. Two common methods are the DJ maximum height (24) and the reactive strength index (19,52). If the maximal jump height were the objective, the method applied in this study would not be recommended for moderately trained athletes, because they may not be able to reach the same height as that of their maximal CMJ. However, if high muscle activation is the objective, Ishikawa et al. (23) showed that a decrease of 10 cm from the optimal height provided the highest activation of the gastrocnemius and vastus medialis.
The greater power achieved with hurdle jumps could be related to the conscious effort to extend the legs in an attempt to regain foot contact as fast as possible before the next hurdle jump. In contrast, during DJs, subjects and athletes essentially “fall” and then contract (with assistance from stretch reflex) resulting in a jump. The conscious leg extension during hurdle jumps may partially contribute to the higher forces measured with the hurdle jumps compared with those of the DJs.
The force curves obtained in this study were similar to those in other studies (27,33). When comparing heel-toe and FORE landing techniques, Kovacs et al. (27) concluded that the heel-toe landing technique was typical of long, triple, and high jumps meanwhile the forefoot landing was typical with other activities that must attenuate the shock force. This attenuation of shock force allows the athlete to develop more power in the following jump.
The contact times in Bobbert et al. (6) and this study were similar (364–400 milliseconds). This long contact time has been attributed to the researcher's instructions that generated a greater knee angle. In another study, Bobbert et al. (7) changed the drop height to 60 cm. The results showed a typical 2 peak force curve suggesting that the height was too much for the subjects' training background. Furthermore, Young et al. (53) analyzed the DJ from 3 different drop heights (maximum height: 60 cm) and reported that as the drop height increased, the percentage of the heel used during contact increased as well.
Hurdle jump preferred and FORE techniques demonstrated a higher RFD than all DJ techniques did. Only the hurdle jump FLAT showed lower values than DJ preferred and FORE technique did. The rationale for these results could be that when contacting the ground first with the heel or a flat foot, there is an impediment for high mechanical power production because of the necessity of balancing the body, and this action increases the contact time and diminishes gastrocnemius and rectus femoris activation. A functional benefit of the FORE strategy is to minimize contact time and produce high mechanical power. In DJs, the FORE strategy is effective if the optimal drop height is implemented. The increased mechanical power could be attributed to the elastic energy accumulated in the stretched plantar flexors during eccentric phase.
However, not all jumps, which use a FLAT or heel-toe technique, have low mechanical power. For example, during a long jump, Luhtanen and Komi (29) reported that a nonelite athlete generated 2,001 N (126 W·kg−1), whereas an elite athlete generated 3,508 N (160 W·kg−1) at the takeoff even when they used a heel-toe technique. Perttunen et al. (37) assessed the vertical force during a triple jump and reported 7,945; 10,624; and 9,056 N for the braking forces and 2,535; 2,680; and 2,491 N for the propulsion forces during the hop, step, and jump, respectively.
How was it possible to produce a high level of mechanical power in the aforementioned studies when this study established that the FLAT technique diminished the RFD? The horizontal velocity during these jumping techniques is extremely high. In the long jump, the takeoff release velocity was 8.40 and 7.09 m·s−1 for elite and average athletes, respectively, and for the triple jump, Perttunen et al. (37) reported 8.65 m·s−1 for the last 5 m. With this horizontal velocity, the athlete's center of mass passes over the point of support very quickly and allows the system to behave like a spring (18). The same principle can be applied for the difference between hurdle jump and DJ in this study.
Only 4 studies have investigated repeated forward jumps over hurdles (8,41,44,49). None of these investigations studied different types of landings. This study is the only study to demonstrate decreased power when employing a FLAT style landing technique with hurdle jumps. Ruben et al. (41) reported an average of 3,373 N for hurdle jumps without clarifying the landing technique (hurdle height 65.2 cm). As he used the instruction to “jump as fast as possible,” it is likely the subjects used the FORE technique. This study generated an average of 4,880 N when using the FORE technique (hurdle height 34.4 cm). The difference may be based on the concept that when the hurdle height is too high the force and power decreases while the contact time increases (8).
In this study, DJ FORE showed the highest vertical force of all DJ landing techniques (3,633 N). This force is less than those produced by all hurdle landing techniques even though the objective of the DJ is vertical height compared with forward hurdle jumps, which must combine vertical and horizontal forces. Hurdle jumps showed more vertical force when using FORE and preferred landing techniques even when part of the force is produced in a horizontal direction.
The degree of muscle stiffness is an important factor when performing powerful movements. Gollhofer et al. (22) reported that during the first 40 milliseconds of the DJ ground contact, muscle length changes were not controlled by neuronal activation. Therefore, during this early phase, the muscle stiffness must be controlled by muscle preactivation. Dietz et al. (16) showed that the EMG activity before ground contact represents an action with a central command as well. The FLAT technique had the least prelanding gastrocnemius EMG activity. It was possible that during a FLAT landing, the supraspinal centers anticipated that the objective of the landing was not to produce the greatest amount of force in anticipation of another movement. This is supported by the EMG preactivation level of tibialis anterior where DJ and HJ FLAT techniques showed the highest level. Hence, the FLAT technique is the most optimal for the absorption of force.
Meanwhile, hurdle jump FORE and preferred and DJ FORE and preferred techniques showed at least a 50% higher EMG preactivity than FLAT techniques. Dietz et al. (15) and Gollhofer and Schmidtbleicher (21) have reported that if an activated muscle is stretched forcefully, high stretch reflex activation should be expected. To increase mechanical power in activities, which show a flight phase before contact, the plyometric training should include activities with a FORE landing technique.
Because the biceps femoris is a hip extensor that helps in the forward movement and the rectus femoris is a knee extensor, all the hurdle jumps showed the highest EMG level, and again this confirms the importance of preactivity to generate muscle stiffness.
The results showed that the hurdle jump FORE technique generated the highest mechanical power compared with all the DJ and hurdle techniques. This high muscular power was possible because this technique had the highest muscle preactivation before landing and the highest muscular stiffness during the eccentric phase. This was corroborated with the EMG activity during pre and eccentric phases. In addition, hurdle FORE generated the highest RFD. A possible limitation of the study was the lack of experience with DJ compared with hurdle jumps and CMJs. However, all the subjects were trained and coordinated athletes who were given a rigorous familiarization session with all jumping techniques.
The present results suggest that jumping forward over hurdles and landing with the forefoot seems to be an optimal technique for developing muscular power. Sport and strength and conditioning coaches must always consider the concept of training specificity (4). If the sport involves rapid SSC type movements with translational movements (i.e., forwards, sideways) then comparable training exercises should be incorporated. Although the CMJ can provide a specific training stress to the athlete, it does not emphasize such a rapid SSC. The DJs do provide a rapid SSC but do not always enlist such repetitive stretch shortening. Many sports involve repetitive or consecutive movements. Thus, although a great variety of plyometric activities should be used in training, hurdle jumps provide a highly training-specific exercise environment. In addition, if the coach can emphasize a particular landing technique, then different physiological factors will be emphasized. For example, to emphasize shorter contact times, higher VGRF, RFD, and leg stiffness, forefoot techniques should be employed, whereas FLAT techniques garner greater muscle activation of muscles such as the rectus femoris and tibialis anterior. Hence, based on the shorter contact time, higher VGRF, RFD, leg stiffness, and greater prelanding muscle activation, the FORE hurdle techniques would provide the best physiological training stimulus for repeated power and speed activities. This would not nullify the implementation of FLAT jump techniques for sports that involve movements such as CMJs (i.e., basketball and volleyball).
1. Arampatzis A, Schade F, Walsh M, Bruggemann GP. Influence of leg stiffness and its effect on myodynamic jumping performance. J Electromyogr Kinesiol 11: 355–364, 2001.
2. Aura O, Viitasalo J. Biomechanical characteristics of jumping. Int J Sport Biomech 5: 89–98, 1989.
3. Behm DG, Button DC, Butt JC. Factors affecting force loss with prolonged stretching. Can J Appl Physiol 26: 261–272, 2001.
4. Behm DG, Sale DG. Velocity specificity of resistance training. Sports Med 15: 374–388, 1993.
5. Bencke J, Naesborg H, Simonsen E, Klausen K. Motor pattern of the knee joint muscles during side-step cutting in European team handball. Scand J Med Sci Sport 10: 68–77, 2000.
6. Bobbert MF, Huijing PA, van Ingen Schenau GJ. Drop jumping. I. The influence of jumping technique on the biomechanics of jumping. Med Sci Sports Exerc 19: 332–338, 1987.
7. Bobbert MF, Huijing PA, van Ingen Schenau GJ. Drop jumping. II. The influence of dropping height on the biomechanics of drop jumping. Med Sci Sports Exerc 19: 339–346, 1987.
8. Cappa DF, Behm DG. Training specificity of hurdle vs. countermovement jump training. J Strength Cond Res 25: 2715–2720, 2011.
9. Cavanagh PR, Lafortune MA. Ground reaction forces
in distance running. J Biomech 13: 397–406, 1980.
10. Chelly MS, Ghenem MA, Abid K, Hermassi S, Tabka Z, Shephard RJ. Effects of in-season short-term plyometric
training program on leg power, jump- and sprint performance of soccer players. J Strength Cond Res 24: 2670–2676, 2010.
11. Cohen J. Statistical Power Analysis for the Behavioral Sciences (2nd ed.). Hillsdale, NJ: Lawrence Erlbaum, 1988.
12. Cormack S, Newton R, McGuigan M, Doyle T. Reliability of measurements obtained during single and repeated countermovement jumps. Int J Sport Phys Perf 3: 131–144, 2008.
13. Cormie P, McGuigan MR, Newton RU. Developing maximal neuromuscular power: Part 2—training considerations for improving maximal power production. Sports Med 41: 125–146, 2011.
14. Debnam M. Plyometric
: Training for power. Mod Athlete Coach 45: 5–7, 2007.
15. Dietz V, Quintern J, Berger W. Corrective reactions to stumbling in man: Functional significance of spinal and transcortical reflexes. Neurosci Lett 44: 131–135, 1984.
16. Dietz V, Schmidtblecher D, Noth J. Neuronal mechanisms of human locomotion. J Neurophysiol 42: 1212–1222, 1979.
17. Ebben WP, Simenz C, Jensen RL. Evaluation of plyometric
intensity using electromyography
. J Strength Cond Res 22: 861–868, 2008.
18. Farley CT, Gonzalez O. Leg stiffness and stride frequency in human running. J Biomech 29: 181–186, 1996.
19. Flanagan E, Comyns T. The use of contact time and the reactive strength index to optimize fast stretch-shortening cycle
training. Strength Cond J 30: 32–38, 2009.
20. Gollhofer A, Schmidtbleicher D. Muscle Activation Patterns of Human Leg Extensors and Force-Time Characteristics in Jumping Exercises Under Increased Stretching Loads. Amsterdam, Netherlands: Free university Press, 1988. pp. 143–147.
21. Gollhofer A, Schmidtbleicher D. Stretch reflex responses of the human M. Triceps surae following mechanical stimulation. In: Proceedings of the XII International Congress of Biomechanics. Gregor R.J., Zernicke R.F., Whiting W.C., eds. Los Angeles, LA: University of California, 1989. pp. 219–220.
22. Gollhofer A, Strojnik V, Rapp W, Schweizer L. Behaviour of triceps surae muscle-tendon complex in different jump conditions. Eur J Appl Physiol Occup Physiol 64: 283–291, 1992.
23. Ishikawa M, Niemela E, Komi PV. Interaction between fascicle and tendinous tissues in short-contact stretch-shortening cycle
exercise with varying eccentric intensities. J Appl Physiol 99: 217–223, 2005.
24. Komi PV. Physiological and biomechanical correlates of muscle function: Effects of muscle structure and stretch-shortening cycle
on force and speed. Exerc Sport Sci Rev 12: 81–121, 1984.
25. Komi PV, Bosco C. Utilization of stored elastic energy in leg extensor muscles by men and women. Med Sci Sports Exerc 10: 261–265, 1978.
26. Komi PV, Gollhofer A. Stretch reflexes can have an important role in force enhancement during SSC exercise. J Appl Biomech 13: 451–460, 1997.
27. Kovacs I, Tihanyi J, Devita P, Racz L, Barrier J, Hortobagyi T. Foot placement modifies kinematics and kinetics during drop jumping. Med Sci Sports Exerc 31: 708–716, 1999.
28. Leukel C, Taube W, Lorch M, Gollhofer A. Changes in predictive motor control in drop-jumps based on uncertainties in task execution. Hum Mov Sci 31: 152–160, 2012.
29. Luhtanen P, Komi PV. Mechanical power and segmental contribution to force impulses in long jump take-off. Eur J Appl Physiol 41: 267–274, 1979.
30. McBride JM, Triplett-McBride T, Davie A, Newton RU. The effect of heavy- vs. light-load jump squats on the development of strength, power and speed. J Strength Cond Res 16: 75–82, 2002.
31. McLellan C, Lowell D, Gass G. The role of the rate of force development during a countermovement jump performance. J Strength Cond Res 23: 379–385, 2010.
32. McMahon TA, Valiant G, Frederick EC. Groucho running. J Appl Physiol 62: 2326–2337, 1987.
33. Mero A, Komi P. EMG, force and power analysis of sprint-specific strength exercises. J Appl Biomech 10: 1–13, 1994.
34. Miller M, Herniman J, Ricard M, Cheatham C, Michael T. The effects of a 6-week plyometric
training program on agility. J Sport Sci Med 5: 459–465, 2006.
35. Nigg B, Denoth J, Neukomm P. Quantifying the load on the human body: Problems and some possible solutions. In: Biomechanics VII. Morecki A., Fidelus K., Kedzior K., Wit A., eds. Baltimore, MD: University Park Press, 1981. pp. 88–99.
36. Peng HT. Changes in biomechanical properties during drop jumps of incremental height. J Strength Cond Res 25: 2510–2518, 2011.
37. Perttunen JO, Kyrolainen H, Komi PV, Heinonen A. Biomechanical loading in the triple jump. J Sports Sci 18: 363–370, 2000.
38. Potach D, Chu D. Plyometrics training. In: Essentials of Strength Training and Conditioning (3rd ed.). Baechle T., Earle R., eds. Champaign, IL: Human Kinetics, 2008.
39. Ronnestad BR, Kvamme NH, Sunde A, Raastad T. Short-term effects of strength and plyometric
training on sprint and jump performance in professional soccer players. J Strength Cond Res 22: 773–780, 2008.
40. Ruan M, Li L. Approach run increases preactivation and eccentric phases muscle activity during drop jumps from different drop heights. J Electrom Kinesiol 20: 932–938, 2010.
41. Ruben RM, Molinari MA, Bibbee CA, Childress MA, Harman MS, Reed KP, Haff GG. The acute effects of an ascending squat protocol on performance during horizontal plyometric
jumps. J Strength Cond Res 24: 358–369, 2010.
42. Schmidtbleicher D. Training for power events. In: Strength and Power in Sport (1st ed.). Komi P.V., ed. Oxford, United Kingdom: Blackwell, 1992. pp. 381–395.
43. Schmidtbleicher D, Gollhofer A. Neuromuscular studies to determine individual load intensity for a jump. Leistungssport 12: 298–307, 1982.
44. Smith JP, Kernozek TW, Kline DE, Wright GA. Kinematic and kinetic variations among three depth jump conditions in male NCAA division III athletes. J Strength Cond Res 25: 94–102, 2011.
45. Taube W, Leukel C, Lauber B, Gollhofer A. The drop height determines neuromuscular adaptations and changes in jump performance in stretch-shortening cycle
training. Scand J Med Sci Sports 22: 671–683, 2012.
46. Turki O, Chaouachi A, Drinkwater EJ, Chtara M, Chamari K, Amri M, Behm DG. Ten minutes of dynamic stretching is sufficient to potentiate vertical jump performance characteristics. J Strength Cond Res 25: 2453–2463, 2011.
47. Verkhoshansky Y. Means and methods: resistance exercises. In: Special Strength Training: Practical Manual for Coaches (1st ed.). Verkhoshansky Y., Verkhoshansky N., eds. Rome, Italy: Verkhoshansky SSTM, 1995. pp. 15–31.
48. Verkhoshansky Y, Siff M. Supertraining (1st ed.). Rome, Italy: Verkhoshansky SSTM, 2006. pp. 46–98.
49. Viitasalo JT, Hamalainen K, Mononen HV, Salo A, Lahtinen J. Biomechanical effects of fatigue during continuous hurdle jumping. J Sports Sci 11: 503–509, 1993.
50. Voigt M, Simonsen EB, Dyhre-Poulsen P, Klausen K. Mechanical and muscular factors influencing the performance in maximal vertical jumping after different prestretch loads. J Biomech 28: 293–307, 1995.
51. Walsh M, Arampatzis A, Schade F, Bruggemann GP. The effect of drop jump starting height and contact time on power, work performed, and moment of force. J Strength Cond Res 18: 561–566, 2004.
52. Wilson GJ, Wood GA, Elliot BC. Optimal stiffness of series elastic component in a stretch-shorten cycle activity. J Appl Physiol 70: 825–833, 1991.
53. Young W, Pryor JF, Wilson G. Effect of instructions on characteristics of countermovement jump and drop jump performance. J Strength Cond Res 9: 232–236, 1995.
54. Young W, Wilson G, Byrne C. A comparison of drop jump training methods: Effects on leg extensor strength qualities and jumping performance. Int J Sports Med 20: 295–303, 2000.
55. Young WB, MacDonald C, Flowers MA. Validity of double- and single-leg vertical jumps as tests of leg extensor muscle function. J Strength Cond Res 15: 6–11, 2001.
Keywords:Copyright © 2013 by the National Strength & Conditioning Association.
plyometric; stretch-shortening cycle; electromyography; reaction forces; landing technique